A fuel cell is a kind of electric power supply capable of generating electric energy by electrochemically oxidizing a fuel such as hydrogen or methanol, and an intense interest has been shown towards the fuel cell, as a clean energy supply source, recently. Particularly, it is expected that a polymer electrolyte fuel cell is widely used as a distributed power generation facility of comparatively small scale, and a power generator of mobile bodies such as automobile and marine vessel, because of such high standard operation temperature as about 100° C. and high energy density. Also, an intense interest has been shown towards the polymer electrolyte fuel cell as a power supply of portable mobile equipment and a portable device, and it is expected to install the polymer electrolyte fuel cell in a cellular phone and a personal computer in place of a secondary cell such as nickel-hydrogen cell or lithium ion cell.
In the polymer electrolyte fuel cell, an intense interest has been shown towards a direct methanol type fuel cell in which methanol is directly supplied as a fuel (hereinafter, referred to as DMFC), in addition to a conventional polymer electrolyte fuel cell (hereinafter, referred to as PEFC) using a hydrogen as a fuel. DMFC has such an advantage that the fuel is liquid and no reformer is used and, therefore, energy density is high and an operating time per one fueled of the portable device is very long.
In the fuel cell, anode and cathode in which the reaction capable of generating electricity occurs, and a polymer electrolyte membrane using as a proton conductor between the anode and the cathode constitute a membrane electrode assembly (hereinafter abbreviated to MEA) and a cell comprising separators and MEA interposed between the separators is formed as a unit. The polymer electrolyte membrane is mainly composed of the polymer electrolyte material. The polymer electrolyte material is also used for a binder of an electrocatalyst layer or the like.
As required properties of the polymer electrolyte membrane, high proton conductivity is exemplified, first. Also, since the polymer electrolyte membrane functions as a barrier which prevents a direct reaction between a fuel and oxygen, low fuel permeability is required. Particularly, in a polymer electrolyte membrane for DMFC in which an organic solvent such as methanol is used as the fuel, methanol permeation is referred to as methanol crossover (hereinafter sometimes abbreviated to MCO) and causes a problem such as decrease in cell output and energy efficiency. As other required properties, resistance to solvents is also an important property as long-term durability against a high concentration fuel in DMFC in which the high concentration fuel such as methanol is used. Other required properties include chemical stability for enduring a strong oxidizing atmosphere during operation of a fuel cell, and mechanical strength and physical durability for enduring thinning and cycling of swelling and drying.
As the material of the polymer electrolyte membrane, NAFION® (manufactured by DuPont Co.) as a perfluorosulfonic acid-type polymer has widely been used. NAFION® is very expensive because it is prepared through multi-step synthesis, and also has a problem that fuel crossover is large because of its cluster structure. Also, there were problems that mechanical strength and physical durability of the membrane formed by swelling and drying are lost because of poor resistance to hot water and poor resistance to hot methanol, and that it cannot be used at high temperature because of low softening point, and a problem such as waste disposal after use and a problem that it is difficult to recycle the material.
Furthermore, there was a problem that since proton conductivity depends on a water content of the membrane, it is necessary to maintain a high humidity condition to exert high power generation performance as a fuel cell and a load of a humidifier is increased. Also, below freezing point, there was also a problem that proton conductivity is largely reduced because water in a conducting membrane concerning conductivity is frozen and therefore power generation becomes impossible.
To solve these problems, some studies on a polymer electrolyte material containing a hydrocarbon-type polymer of a nonperfluoro-type polymer as a base have been made. As a polymer structure, particularly intensive study on an aromatic polyether ketone and an aromatic polyethersulfone has been made in view of heat resistance and chemical stability.
For example, there have been proposed a sulfonated compound of a poorly-soluble aromatic polyetherether ketone (examples thereof include such as VICTREX® PEEK®, manufactured by VICTREX Co.) which is an aromatic polyetherketone (see, for example, non-patent document 1), polysulfone in a narrow sense as an aromatic polyethersulfone (hereinafter sometimes abbreviated to PSF) (examples thereof include UDEL P-1700, manufactured by BP Amoco Polymers, Inc.), a sulfonated compound of polyethersulfone (hereinafter sometimes abbreviated to PES) (examples thereof include Sumikaexcel® PES, manufactured by Sumitomo Chemical Co., Ltd.) in a narrow sense (see, for example, non-patent document 2) and the like, but there were a problem that if a content of the ionic group is increased in order to enhance the proton conductivity, a prepared membrane swells and therefore fuel crossover such as methanol or the like is large, and a problem that since the polymer electrolyte material is low in a cohesive force of a polymer chain, stability of a polymer higher-order structure is insufficient and mechanical strength and physical durability of a prepared membrane are insufficient.
Also, in the sulfonated compound (for example, non-patent documents 1 and 2) of an aromatic polyetherketone (hereinafter sometimes abbreviated to PES) (examples thereof include VICTREX PEEK-HT, manufactured by VICTREX Co.), there was a problem that because its crystallinity is high, a polymer having the composition of low density of a sulfonic acid group becomes insoluble in a solvent, resulting in poor processability because of a remained crystal moiety. To the contrary, when the density of the sulfonic acid group is increased so as to enhance processability, the polymer is not crystalline and drastically swells in water and, therefore, the membrane thus formed not only shows large fuel crossover but also is insufficient in mechanical strength and physical durability.
Furthermore, there have been proposed an aromatic polyethersulfone block copolymer (for example, patent document 3) and an aromatic polyetherketone block copolymer (for example, non-patent document 3 and patent document 4). However, also in these copolymers, there was a problem that polymers are brittle and low in structural stability since these polymers use an amorphous polymer such as a PES-type polymer or a PEK-type polymer having a bulky side chain as a base structure because of solubility constraint and membranes prepared are inferior in dimensional stability, mechanical strength and physical durability.
As described above, the polymer electrolyte material according to prior art is insufficient as a measures for improving economic efficiency, processability, proton conductivity under the conditions of a low humidity or a low temperature, fuel crossover, mechanical strength and therefore long-term durability, and there has never been obtained an industrially useful polymer electrolyte material for a fuel cell.    Non-Patent Document 1: “Polymer”, 1987, vol. 28, 1009    Non-Patent Document 2: Journal of Membrane Science, 83 (1993) 211-220    Non-Patent Document 3: “Polymer”, 2006, vol. 47, 4132    Patent Document 1: Japanese Unexamined Patent Publication (Kokai) No. 6-93114    Patent Document 2: Published Japanese Translation No. 2004-528683 of the PCT Application    Patent Document 3: Japanese Unexamined Patent Publication (Kokai) No. 2003-31232    Patent Document 4: Published Japanese Translation No. 2006-512428 of the PCT Application